Light sensitive cell



Sept.A 22, 1936.

E. PRAEToRlUs i-:r Al. 2,055,017

LIGHT SENSITIVE CELL Filed DSC. 28, 1932 Wil,

faim/d Pme/0025 ATTORNEY' Patented Sept. 22,A 1936 LIGHT SENSITIVE CELL Edward Praetorius and John E. Stewart Los Angeles, Calif.

Application December 28, 1932, Serial No. 649.144

5 (llaimil.` (Cl. 201-63) This invention relates to a light sensitive cell, and especially to a type of cell known as a selenium cell, for converting variations of light intensities into variations in current flow.

Since the theory of operation of such devices is very well known, it is unnecessary to detail it here. Such cells are in common use for example ior sound reproduction from motion picture ilms, and for burglar alarm systems, or the like.

In cells of the prior art, it has been common to provide a grid of suitably sensitized selenium between two metal contacts, formed for example of platinum attached to a glass backing. ,In such cells the grid of selenium acts as a dividing strip between the contacts, whereby a considerable area of contact from the selenium to each contact is secured. Y v

It is one of the objects of this invention to provide an improved form of cell of this character.

It is another object of this invention to provide .a cell construction that is rugged, and in which the electrical connection between the selenium and the cooperating contacting metal, is rendered more reliable and permanent.

In many instances, such grids of selenium are interposed in the path of light rays passing through a sound track on a film. In order that proper and correct reproduction be secured, it is essential (as is well understood) that only a very narrow portion of the sound track be active at a time. This has necessitated the use of very narrow slits to pass the light to the cell, as well as the use of an optical system for directing the light so transmitted, onto the active material. I

Such narrow slits have usually been provided in plates of opaque material. Since extremely narrow slits are needed for correct sound deiinition (of the order of a small fraction of one onethousandths of an inch, when`used with lms of a size suitable for amateur use) it is apparent that such slits can be easily clogged up by iine particles of dust settling upon the inner surfaces of the slit, which are very deep as compared with the width of the slit.- Such dust is apt to be derived from the iilm surface, which runs close to the slit. To

` overcome this, it has been proposed to fill the slit with some transparent material, such as glass or quartz. However, such schemes have disadvanl tages, such as optical aberrations introduced into and has other objects which may be made more easily apparent from a consideration of one embodiment of the invention. For this purpose there Iis shown a. form in the drawing accompanying and forming part of the present specication. This form shall now be described in detail, which illustrates the general principles of the invention; but it is to be understood that this detailed description is not to be taken in a limiting sense, since the scope of the invention is best defined by the appended claims.

Referring to the drawing:

Figure 1 is an enlarged section of a complete cell embodying the invention;

Fig. 2 is an enlarged section taken along plane 2-2 of Fig. 1;

Fig. 3 is a front view oi the cell, taken along plane 3-3 of Fig. 1, the scale being somewhat smaller than that o! Fig. 1, but nevertheless considerably enlarged; and

Fig. 4 is a very greatly enlarged sectional view of a portion of the cell.

The photosensitive cell has as a main support a translucent member I, which may be in the form of a thin glass disc. For sound reproduction purposes, the member I, can be about three-fourths of an inch in diameter and about .050 of an inch thick. However, these dimensions may be varied.

On the back side of the translucent member or support I, selenium 2 or other suitable prepared light sensitive material is supported. In this way, light passing through the translucent member I will aect the selenium 2. In order to secure a large response, the selenium 2 is deposited and supported on\ the glass plate I in a manner now to be described.

First of all, in order to provide contact members for the selenium material 2, one side of disc I is covered with athin layer of metal which has a coeicient oi' expansion substantially the same as the support I. This metal may be a thin layer 20 of platinum which has been suitably fused to the glass I. This fusion to the glass can he accom? plished by any well known process, and the metal layer can be very thin; preferably of the order of .G0001 of an inch or less. Then by the aid oi' an engraving machine or the like, the platinum layer 2i) is divided intotwo contact members, having the form indicated by reference characters 3 and l in Fig. 2. 'I'he thickness of these contact members is very greatly exaggerated in Figs. 1 and 4. By the aid of the engraving tool, the platinum layer is divided by a grid-like strip, where the metal has been removed. At this stage of the process, the glass I is visible through the convolutions of the grid. This grid occupies an area at the center of the disc of about a quarter of an inch square, and the width oi the strip can be of the order of one one-thousandth of an inch. The lines forming the grid can be spaced so close together as to form one hundred lines in a fourth of an inch. This spacing, as well as the width of the cut away strip, is greatly exaggerated for clearness in Fig. 2.

It is apparent that suitably prepared selenium could then be deposited in the convolutions of the grid, forming a long convoluted strip of active material between the platinum contact members.

These contact members would then be in electrical contact with opposite sides of the strip or selenium.

However, it has been found that a much better contact can be secured by interposing between the platinum members and the selenium, a layer of metal that is capable of adhering strongly to the selenium as well as to the platinum. One such'metal is gold. Accordingly, a very thin layer of gold 5 can be deposited over all of the exposed sides of contact members 3 and d. This can be done by an electroplating process, by the aid of which an extremely tenuous film of gold can be deposited. For this purpose, leads and 'i are first securely soldered respectively to the platinum members d and 3. These leads can also be used as terminals for the completed cell. For the purpose of electroplating the gold, these terminals are temporarily connected together, and the two leads with the two contact members 3 and li can form the cathode of an electroplating circuit.

As shown most elearly in Fig. 4, the electrolytically deposited gold layer 5 covers all of the exposed surfaces of the platinum. Its thickness is very greatly exaggerated in Fig. 4. Next the selenium material 2, suitably sensitized, can be laid over the gold layer 5 and in the grid spaces formed between the interlacing portions of the platinum members 3 and il.

Furthermore, we prefer to provide some granular hygroscopicl material 8 immediately back of the sensitive layer 2. Such hygroscopic material for example can be calcium chloride or phosphor-4 us pentoxide. This can be held in place as by the aid of a Celluloid cap 9 attached to the back of the disc l as by the aid of a Wax seal l0. In this way, deterioration of the selenium layer 2 by climatic or-humidity conditions is effectively prevented.

As thus far described, it is apparent that the contact members 3 and d with the overlaid tenuous film of vgold are in intimate electrical con- `tact with opposite sides of a selenium grid formed in the grooves between the contact members. For sound reproduction it has been considered essential when cells oi this character are used, to provide an optical system, whereby an extremely narrow beam of light passing through the sound track will aiect the selenium cell at any instant. In commercial sized lms, that is 35 mm. llms, the beam of light should not be greater than one one-thousandth of an inch wide; and in work utilizing the smallersized lms, that is 16 mm. films, the beam of light should be not more than one-quarter as wide; that is of the order of onequarter of one one-thousandth of an inch.

This eect has been accomplished in the pa'st by interposing a very narrow slit between the film and the cell. This slit has been formed of metal plates having a thickness much greater than the width of the slit. It has been found diflicult to goeder? keep such a silt clean from dust, and for this reason it has been common to iill the slit with some transparent material.

By the aid of this invention, such a structure is obviated, and a compact slit forming device is provided in connection with support l. For this purpose, a slit is formed by the 'aid of very thin opaque members, which provide no foothold for the reception or retention of dust particles. Thus when the lm is passed adjacent the front of the glass disc l, any dust that may tend to settle on the slit will drop oir, as the edges of the slit are too narrow for holding them in place.

To accomplish this result, there is deposited on the front side of the glass disc l, an extremely thin, opaque layer ll of platinum or its equivaI lent. This layer of platinum can have a thickness of the order of .00901 of an inch or less, and is fused to the glass. Then a very thin slit i2 is cut through this platinum layer, and preferably in a direction transverse to the convolutions of the selenium grid. This slit 2 can be of any desired degree of iineness, such as one onethousandth of an inch for commercial iilms, or one-quarter that width for the amateur sizes.

It is apparent that with such an arrangement, the cell can be placed quite close to a sound track, and there is no need of the interposition of any optical system. The light passing through slit l2 and the disc i, may spread to some extent in a direction transverse to the slit, as by diiraction; but this merely results in rendering a larger portion of the exposed selenium area eective,

without reducing the fineness of the definition.

The translucent support i with its adherent members can be appropriately supported to form a cell assembly, in any desired way. One manner of supporting it is illustrated in Fig. 1.

Thus a tubular casing i3 is shown, having a iiange lf. A fibre or yielding washer i6 is placed inside of the tubular casing i3 and on top of this washer is placed the cell structure including the support l. Then a tube i6 of rubber or other yielding material is slipped inside of the casing i3 and is held in place by friction. It engages the cell near the outer edge of the cell and serves as a shock absorbing medium to guard against rough usage. An insulation cover Il is then pressed into the casing i3 and is frictionally held therein; by the inner wall of the casing. This insulation cover can'be of bakelite or equivalent material. It can carry the binding posts i8 and l 9, to which leads 6 and l may be soldered. These posts extend externally of the casing for appropriate connection in the sound circuit. When pressed into place as shown in Fig. l, the cover il slightly compresses the resilient tube i6 against the dise l, thereby holding this disc and its associated parts in tight relationship to casing i3.

It is apparent that the present structure provides an optical slit supported on one side or surface of a glass disk, and a selenium cell supported on the other or opposite side or surface of the same glass disk. Thus there is produced a one piece combination in which the optical slit is not subjected to any electrical stresses.

This is an important consideration. When it is attempted to use a singley selenium line cell to obviate an optical system, the line of selenium must be very fine, of the order of .001 inch in.

width. The slit in which this selenium is placed is dened by adherent metal layers bridged by aosaoir centrated on a `very narrow area and width of selenium. Accordingly, there is a likelihood of electrical breakdown of the cell across the slit, as well as a likelihood of the production of microscopic noises.

In the present construction described herein, these disadvantages 'are entirely overcome. The grid 2 presents ample area to terminal plates 3 and l to withstand an electrical breakdown. The optical slit l2 can be made as narrow as needed, as no selenium is associated therewith, nor is there any electrical potential across its sides. The slit l2 performs the sole function ,of an optical system or mask, while the selenium on the other side of the glass disk i performs the sole function of a variable resistance photo-cell.

In the claims, the term selenium material is intended to include not only selenium, but compounds thereof that exhibit light sensitive properties similar to selenium.

Also, the term optical slit in the claims is intended to refer to a slit used in sound reproduction of apparatus of such narrow proportions as to obviate the necessity of a lens system to confine the illumination of the light sensitive cell to the requisite narrow area.

We claim:

1. In a cell of the character described, a transparent insulation support, a t'wo-part metal layer,

fused to the support and spaced apart to dene a narrow strip between the parts, selenium material in the strip and in contact with both parts, and a two part metal layer fused to the other side of the support and defining a narrow slit.

2. In a cell of the character described, a transparent insulation support, a two-part platinum layer, fused to the support and spaced apart to dene a narrow strip between the parts, a thin layer of gold disposed over the two-part metal layer, and selenium in the strip and in contact with the thin layer.

3. In a cell of the character described, a transparent insulation support, means forming a pair of spaced platinum contact members fused to the support, a thin layer of gold disposed over both members, and selenium bridging the members.

4. In a cell of the character described, a glass support, a pair of spaced platinum contact members fused to the support, a thin layer of gold disposed over the members, and selenium bridging the members.

5. In a cell of the character described, a glass support, a pair of spaced platinum contact members fused to the support, a thin layer of gold electrolytically deposited over the members, and selenium in contact with said thin layers and bridging the members. 

